The conversion of chemically stored energy into useful work has been the goal of engine designers since the creation of internal combustion engines utilizing the Otto cycle. As is well known, the most widely available devices nowadays for converting fossil fuel energy into rotational power are the Otto and Diesel cycle reciprocating internal combustion engines and the Brayton cycle gas turbines.
The reciprocating or piston-type internal combustion engines typically demonstrate relatively low fuel efficiency. Indeed, typically less than 20% of the chemical energy stored in the fuel is transformed into useful mechanical energy. This relatively low efficiency is imputable to many factors.
For example, complex mechanical systems are required to transform the reciprocating motion of the piston into the rotary motion of the drive shaft. Also, the mechanical properties of the materials used for building conventional combustion chambers significantly limits the allowable temperature and pressure in the combustion chamber and, thus, limits the thermal efficiency of the engine.
Furthermore, with most conventional reciprocating internal combustion engines, the combustion of the fuel occurs at ordinary rates. These ordinary rates of combustion result into prolonged heating of the combustion chamber which produces more degradation at the chamber wall per unit volume of fuel burned than when the fuel is burned at a faster rate. This, in turn, limits the specific power or power-to-weight ratio of the engine.
Rotary engines compared to reciprocating engines significantly reduce the mechanical complexity by eliminating the need to transform the reciprocating piston motion to rotary motion of the drive shaft. The prior art is replete with various types of rotary engines in which the rotor has a plurality of circumferentially spaced combustion chambers formed with ducts to exhaust combustion products in order to provide reaction forces. However, most conventional rotary engines do not provide substantially improved efficiency or power-to-weight ratio over the conventional reciprocating engines probably at least in part because the combustion of fuel still occurs at ordinary rates.
In view of the scarcity and high costs of engine fuels, engine designers and engineers have been grappling mostly with the fundamental problems of exhaust emissions and pollutants and increased fuel economy, yet striving to improve performance in these areas without sacrificing already compromised engine performance and efficiency. Over time, numerous proposals have been set forth to reduce pollution and increase engine and fuel performance. Each has some distinct disadvantage because of its interaction with other engine parameters inherent in the Otto or Diesel cycle engines.
Although engine designing efforts have been directed mostly towards these fundamental problems and, in particular, towards improving the efficiency of the fuel-to-work conversion, other engine parameters may in some situations be considered at least as important. For example, in some specific settings, the so-called specific power or power-to-weight ratio is sometimes considered a crucial design and operational criteria.
For example, it is well known that with various types of vehicles, weight may become a critical factor. Indeed, extra weight in vehicles such as automobiles and airplanes in particular require substantial propulsion power and also reduce maneuverability. When the specific power of the engine is not optimized, the engine itself may constitute an important fraction of the total vehicle weight. Accordingly, many types of vehicles could greatly benefit from a simple propulsion system having a relatively high power-to-weight ratio in order to minimize the overall vehicle mass.
Furthermore, there also exist many situations wherein engines are only required to provide power for limited periods of time. Some of these situations would also greatly benefit from an overall reduction of the engine weight and, hence, optimization of the specific power since the engine may be considered as a “dead mass” for most of the operational cycle.
In the field of aerospace, typical examples of situations wherein optimization of the specific power could prove to be particularly interesting include upholding of the gyroscopic stabilization of satellites, generating power for space weapons and tools, providing power for future human space stations, motorizing some of the mechanisms during spatial machines take-off, providing propulsion for personal flight vehicles and so forth.
In the automotive industry, hybrid vehicles in particular could potentially greatly benefit from an engine able to demonstrate a relatively high specific power. Indeed, in such hybrid vehicles, piston engines are sometimes used as generators and peak power leveler engines to counterbalance the lower power and relatively low autonomy of conventional batteries. Such vehicles would hence greatly benefit from auxiliary engines having a relatively high specific power that could be used as a peak leveler. Also, electrically powered vehicles such as an hydrogen fuel cell electric vehicle could benefit from an engine adapted to act both as a constant or near constant energy storage flywheel and as a peak power supplier when needed such as during acceleration.
In the field of biomechanics various devices such as limb prosthesis or the like could also greatly benefit from a relatively light actuator able to provide relatively short bursts of power. In general, any application wherein there exists a need for a providing peak power for relatively small periods of time and without sacrificing weight criterias could benefit from an engine optimized for specific power.
As is well known, of the currently available energy converters, the so-called gas turbine appears as one of the best design in terms of specific power. Conventional gas turbine engines have generally included a stationary combustion chamber or burner where injected kerosene or other fuel and air from a compressor is mixed and burned. The burnt gas may pass through a duct which directs it against the blade of a rotating turbine blade wheel that delivers through its shaft. High efficiency and high power output from such engines depend on the use of gas jets of high energy being directed at the turbine blade wheel. However, if the jet energy increases to a substantially high level, large thermal and mechanical stresses are imposed on the blade which may cause mechanical failure.
Indeed, relatively large mechanical stresses are produced in available engines by reason of the high rotational speeds for typical turbine blade wheel diameters. When these mechanical stresses are combined with high temperatures imposed by the gas jet, the conditions become close to the limit of strength of the best available turbine blade materials.
Higher gas jet energy can be readily obtained, but they cannot be efficiently used since if the blade wheel is allowed to rotate faster, this causes excessive mechanical stress, while if the gas jet is moving too fast relative to the blade, for example more than about Mach 1, then this causes excessive heating of the blade.
Typically, with conventional gas turbine engines, long cycle life is obtained by using large quantities of extra air to produce gas released at moderate temperature and velocity and by using moderate rotational speed of the turbine blade wheel. The result is relatively low efficiency even for large units and limited power output for a given size of engine.
The mechanical resistance of the materials used for building the gas turbines is hence often considered the limiting factor that hinders or limits the specific power performances of conventional gas turbines. Also, conventional gas turbines are usually not considered as a commercially interesting high specific power propulsion system because of their inherent high costs and complexity.
The need for providing engines capable of demonstrating a relatively high specific power while has been recognized in the prior art. One of the proposed solutions involves the use of shock waves instead of mechanical compressors for compressing the combustible fluid. Since shock waves allow for relatively high rotational speeds, relatively low pressures are required to obtain potentially interesting power values as opposed to conventional turbo-engines requiring relatively large forces because of their relatively low rotational speed.
Various types of thrusters using shock wave combustion to provide thrust are known. For example, ramjets are widely known and have been used primarily in aerospace applications since the 1940. In an aerospace propulsion ramjet, air in ingested into an engine inlet at supersonic speeds caused by the forward motion of an airplane or missile. The air is rammed into a smaller opening between a center-body and the engine side wall generating a series of shock waves. These shock waves compress and decelerate the air to subsonic speeds while, at the same time, dramatically raising working flow pressure and temperature.
The ramjet effect may also be achieved in a stationary platform by passing an accelerated flow of air over raised sections machined on the rim of a rotor disc. Combined with the high rotation rate of the rotor, this produces a supersonic flow relative to the rotor rim. Interaction between the raised sections of the rim which are rotating at supersonic speeds and the stationary engine case creates a series of shock waves that compress the air stream in a manner similar to ramjet inlets on a supersonic missile or aircraft.
Over the years, the combustion chamber configuration, the configuration of ramjet engine inlets, the fuel injection requirements and ignition requirements have been the subject of much studies and technical development. Ramjets have also been experimentally employed to assist in the rotation of helicopter blades about a central shaft.
The prior art has further shown examples of rotary motors using shock wave combustion produced by the so-called ramjet effect. For example, U.S. Pat. Nos. 5,372,005 et 5,709,076 both naming S. P. Lawlor as inventor and issued respectively in 1994 and 1998 disclose both a method and an apparatus for generating power using rotating ramjets which compress inlet air and expand exhaust gases against stationary peripheral walls.
The engine disclosed in the hereinabove mentioned patents is commonly referred to as a ramgen. The ramgen typically includes a pair of thrusters mounted in a diametrically opposed relationship relative to each other on the engine rotor
The tangential thrust produced by the thrusters provides the rotational output power. Although interesting, the ramgen nevertheless suffers from inherent drawbacks. One of these drawbacks relate to the fact that the inlet air of a given thruster may be potentially contaminated by the exhaust air of the other thruster. A large fan positioned upstream relative to the ramgen inlet must hence be used in order to discharge exhaust gases.
Also, ramgen-type engines are mainly designed for the production of electrical and mechanical power at medium size electrical or mechanical power plants. Such medium size mechanical or electrical power plants, typically in the range of 10 to 100 megawatts are typically required in industrial applications including stationary electric power generating units, rail locomotives, marine power systems and the like.
Power plants in this general size range are also typically suited for use in industrial co-generation facilities increasingly employed to service industrial thermo-power needs while simultaneously generating electrical power. Obviously, ramgen-type engines are mainly concerned with the efficient conversion of fuel input to electrical output as opposed to being concerned with specific power. Accordingly, design choices such as the use of only two thrusters inherently limit the ability of ramgen-type engines to provide high specific power.
The prior art has also shown some examples of other types of rotary supersonic combustion chambers. For example, U.S. Pat. Nos. 3,971,209 and 4,199,296 both naming R. S. Chair as inventor and issued respectively in 1976 and 1990 disclose respectively various embodiments of gas generators and gas turbine engines. The structure disclosed in the above-mentioned patents is commonly referred to as a rambine.
The rambine, although somewhat interesting, may be considered as “self-propelled compressor-combustion chamber combination” as opposed to an autonomous engine. Furthermore, the angle of attack at the inlet diffuser of the rambine typically does not coincide with the radial direction of the engine as is the case with conventional compressors. This may potentially limit the overall performance of the rambine.
Accordingly, there exists a need for an improved engine allowing for the conversion of chemically stored energy into output power with relatively high specific power characteristics.